I understand, generally, *how* they got that big but not *why*. What was the evolutionary advantage to their massive size? Is there one? Or are they just big for the sake of being big?
In: Biology
They are just the largest thing still alive. Not so back when they started out. The others just didn’t survive
There’s a big advantage: big animals are hard to kill. There’s a very short list of animals that can hunt a blue whale. In fact that list might just be one creature (orca).
Not being able to be hunted down is a really good advantage 😉
Blue whales got so big because being big helps them eat better. Their food, krill, is tiny and spread out. Bigger whales can take bigger gulps of water and filter out more krill at once. This means they get more food for the effort they put in. Over a long time, the bigger whales were more successful and had more babies, who were also big. So, over millions of years, they gradually evolved to be enormous, not to fight off enemies, but simply to eat more efficiently.
There is no “why.” That’s not how evolution works. There is no plan, there are no ideas being expressed, there are no reasons for this or that. Evolution is random chance. Successful chances survive well in their environment, and are able to reproduce. Unsuccessful chances die off.
1. Partially they’re big because it pays off to be big. No predators for adult blue whales.
2. Partially it’s about efficiency. Have you ever wondered why transport ships are so big? Well. When swimming, the bigger you are the better the ratio is for weight vs the effort to transport that weight. A blue whale utilizes that to be really efficient when it comes to swimming (minimum amount of calories spent per kilo of whale per kilometer), and they use that bulk to basically become a big krill consuming factory that goes from one shoal of krill to another and vacuum up everything and converting that biomass into more whale.
It is important to understand this point. There is no “why”.
It just happened. There are a lot of reasons that being large gives you evolutionary advantage (defined as making it more likely that you have offspring), but there is no reason for any animal existing beyond the effectively random circumstances that caused their ancestors genes to mutate in a way that eventually produced the current animal.
“Why” implies that someone or something made a decision. That did not happen. (Unless you believe the “why” is God. In that case, it is a matter of faith and there is no point discussing as it is not provable one way or another.)
The trend towards larger whale species seems to coincide with Ice Age cycles.
Being so large and having so much more insulation meant that Blue Whales could stay underwater longer, and go deeper meaning that they can continue to feed when the polar ice sheets extended south and their food source krill became more scarce.
Many smaller species of whale went extinct during the Ice Ages leaving the larger species to thrive.
The part that confuses me about whales is that they’re mammals, right? So the biggest sonofabitch in the ocean went onto land long enough to lose gills, then crawled BACK into the ocean for a quick dip that’s lasted the last few dozen million years or however long.
Is that pretty much it?
This is ELI5 and not askscience, but anyone interested in a paper on the topic can find a good one here
[https://www.science.org/doi/full/10.1126/science.aax9044](https://www.science.org/doi/full/10.1126/science.aax9044)
To try to boil it down to ELI5 level….whales benefit from increased energy efficiency the larger they get. For example, oxygen storage gets better as size increases, and movement through the water gets more efficient. However, toothed whale size is limited by the size of prey they can find. Abundant large prey is needed to support large body sizes, because it’s just not efficient to have a big body and individually chase down large numbers of small prey.
Baleen whales avoid this problem by filterfeeding. Instead of eating one prey at a time, they scoop up a swarm of prey animals and eat them all at once. As such, their size isn’t constrained by abundance of large prey, but by abundance of swarms of small prey. And there’s a lot of krill in the ocean. So they could get bigger and bigger and benefit more and more from those size-based efficiencies in diving and movement.
Because they consume tons of water at a time to filter out tiny living creatures called krill, the only way using their method of feeding to get enough krill is to take up huge mouthfuls at a time. Krill are tiny transparent crustaceans looking like tiny shrimp and feed in the southern oceans around Antarctica and in turn are eaten by baleen whales and especially the blue whale. https://youtu.be/RH6tuE1qxHo
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